ABSTRACT Plasmodium vivax has not traditionally been considered in malaria elimination strategies across Africa, due to the assumption that its transmission cannot be supported in the predominantly Duffy-negative population. Nevertheless, there is growing evidence that P.vivax is present across the continent and Duffy-negative individuals can contribute to onward transmission. Data from other elimination settings in Southeast Asia show that P.vivax becomes more prominent as P.falciparum (the dominant malaria species in Africa) declines. This poses a major threat to malaria elimination in Africa, particularly since surveillance and control strategies across the continent are almost exclusively focused on P.falciparum. Mobile pastoralist communities in northwestern Kenya may be especially susceptible to P.vivax infection due to their proximity to and seasonal contact with neighboring Ethiopia, where both P.vivax and Duffy-positive populations are present. They have been moving to and from Ethiopia with their herds for hundreds of years, thus it is possible that 1) these northwestern Kenyan communities harbor higher levels of Duffy antigen than most Kenyan populations and 2) their regular movements represent opportunities for importing P.vivax. Therefore, the pastoralist communities in northwestern Kenya represent a high-value surveillance population for monitoring and mitigating the threat of P.vivax transmission in the region. However, they remain largely outside of routine public health activities due to their isolated and remote nature, and little is known about their health status or malaria risk. We propose to leverage samples and data from a recently completed study in nomadic pastoralists in order to quantify the threat of P.vivax in this specific group and in the region more broadly. The parent study confirmed elevated risk of P.falciparum infection in those traveling with the herds compared to those who remain in the settlement. In addition, we have already identified several P.vivax infections, but the extent and relationship to Duffy antigen are still unknown. In the proposed work, we will first screen >1500 samples for P.vivax infection and describe the prevalence of P.vivax for the first time in this unique population. Then, we will work with Community Health Workers to obtain permission to test participant samples for expression of the Duffy receptor. This new information, coupled with the rich data associated with each sample, will allow us to assess the risk of emerging P.vivax in this group, determine the value of enhanced surveillance and targeted control, and recommend criteria for identifying critical surveillance populations as Kenya moves towards malaria elimination. The findings from this study would have broad application, informing malaria surveillance and control strategies in Turkana, Kenya, and areas across SSA where P.vivax may have an increasing impact as P.falciparum is controlled.